First-generation nuclear power plants utilized metallic, reflective-type insulation for pipes and equipment located inside the containment buildings. It was found that this reflective insulation did not produce the promised reduction in heat loss and resulted in reduced efficiency and increased operating costs. Further, during maintenance shutdowns this fragile metal insulation was damaged by foot traffic and/or handling making it difficult or impossible to reinstall. Since each piece was custom-made, replacements could not be readily obtained during the scheduled shutdown period. In addition, this insulation did not lend itself to easy inspection of pipe welds, which must be performed at regular intervals.
As a result of these problems, nuclear power plants began to replace this metal insulation with removable blankets, particularly in areas where inspection of pipe welds was required. A potential problem arises, however, with use of blanket insulation during a loss of coolant accident (LOCA). During a LOCA, the blankets may be subjected to high energy liquid jets (i.e., subcooled liquid or steamwater mixtures at high pressure) which enter the containment area. If the jet stream impinges on the blankets, the fibrous blanket material may be torn lose generating debris which can clog the protective screen of the emergency core cooling system (ECCS) sump and thereby impair recirculation of water from the sump.
The first blanket system accepted by the United States Nuclear Regulatory Commission consists of a two-inch thick blanket of very light density glass fiber fillers totally encapsulated in a glass fiber cloth envelope. The blankets are used in either single or double layer construction depending upon the pipe temperature. The cloth envelopes are formed with sewn seams and, in the case of double layer construction, the seams in the two layers are staggered with respect to one another. Velcro fastenings are used to attach the blankets to the pipes and/or adjacent blankets. An outer jacket of 26-gauge (0.01875 inch thick) stainless steel fastened by suitcase latches is sometimes used to protect the blankets. These fragile blankets are intended to be completely destroyed in the event of a LOCA with the hope that the residue from the blankets will be small enough so as not to block the sump screen and, if necessary, to pass through the recirculating pump.
Another known insulating blanket used in nuclear containment buildings consists of a glass fiber filler layer having a waterproof sheet of ERCO-SIL 36S (sold by Eastern Refractories Company, Inc., Belmont, Mass.) on its outer surface (i.e., disposed away from the pipe). The remaining sides of the filler layer are covered by glass fiber cloth. Metal clips consisting of stainless steel hog rings connect the cloth to the waterproof sheet. This design is much stronger than the first design and is intended to resist damage during a LOCA.
Other known insulating blankets used in nuclear containment buildings are described in Durgin, W. W., and Noreika, J. F., "The Susceptibility Of Fibrous Insulation Pillows To Debris Formation Under Exposure To Energetic Jet Flows," NUREG/CR-3710 (SAND 83-7008), Alden Research Laboratory and Sandia National Laboratory, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555. A first blanket described therein consists of 16 lb/ft.sup.3 mineral wool with a cover of asbestos cloth coated with one-half mil of Mylar (registered trademark of E. I. DuPont de Nemours & Co., Wilmington, Del.). A second consists of 11 lb/ft.sup.3 needle packed fiberglass layers covered with stainless knitted mesh on one side (close to the pipe) and silicone glass cloth on the other (outer) side. A third consists of 11 lb/ft.sup.3 needle packed fiberglass layers covered with 18 ounce fiberglass cloth. These three blankets were tested for damage and failure (loss of blanket material) under jet streams of up to 65 psi applied at an angle of 45.degree. and 90.degree. . The third blanket exhibited the greatest resistance to damage and failure.
In the field of thermal and acoustic insulation blankets for exhaust systems in gas transfer plants, oil rigs, refineries and the like, it has been suggested to provide an insulation blanket comprising a ceramic fiber core approximately one and one-half inches in thickness, a thin lead sheet adjacent the core for noise insulation, a thin stainless steel foil/fiberglass cloth laminate between the core and insulated object for excluding moisture, a silicone rubber coated fiberglass cloth completely surrounding the core and other layers, and a flexible stainless steel mesh stocking enveloping the entire assembly to provide additional protection (U.S. Pat. No. 4,442,585). These blankets are wrapped tightly around pipes by means of lacing anchors and second layers are provided around the joints to render the edges significantly impermeable to thermal and acoustic radiation.